Wastewater treatment and petroleum/chemical processes result in the emission of a wide assortment of odorous compounds, such as ammonia and organic sulphides, and volatile organic compounds. The stream of contaminant-laden exhaust air from sludge settling tanks, aeration tanks, wet wells, etc. must be treated to reduce odor or target pollutants below human smell recognition threshold levels and to reduce volatile organic compound levels before the airstream is discharged into the atmosphere. Various treatment technologies are currently being used to treat the exhaust air, including: combustion; scrubbing with water, caustics, bleach or other oxidants; filtration through odor filter piles or other media (carbon); dilution with fresh air; and final discharge and dispersion through elevated stacks.
Activated carbon is particularly useful in removing volatile organic compounds and odorous sulphur compounds from exhaust gas. Carbon adsorbs these contaminants. After a period of continuous use, however, carbon becomes exhausted, i.e., it loses its capacity to adsorb, and must be replaced.
A known system for treating exhaust gas using activated carbon involves directing the gas stream into the base of a cylindrical tank A (see FIG. 1). The gas flows upwardly, percolating through a bed B of carbon disposed across the tank, and then is discharged out the top of the tank. As the gas flows upwardly from the base, it first contacts a circular bottom surface C cf the carbon bed B which has a surface area of .pi.d.sub.t.sup.2 /4 (where d.sub.t is the inside diameter of the tank). In an alternative system, carbon beds may be stacked, one above the other, in a dual bed design such that the airstream will split between the beds. In this latter system, however, it is relatively more difficult to replace the carbon and oftentimes there is uneven flow distribution causing one bed to be exhausted before the other.
Practical adsorption capacity of the carbon bed is directly related to the surface area C of the bed and to the thickness of the bed. However, in view of pressure drop considerations, it is desirable that bed thickness be limited, i.e., a thicker bed requires a larger blower and more power to maintain the desired airstream velocity. Generally, a carbon bed thickness of about three feet is considered economical in treating exhaust air. If the air, however, contains significant amounts of contaminants, a further increase in bed thickness may be advisable.
A disadvantage of the tank in FIG. 1 is that adsorption capacity is primarily limited by the diameter of the tank. For high flow rates, the footprint of the tank will get very large. Additional or larger tanks may be constructed, but at higher cost for material and labor. The availability of land area poses another major problem for the location of additional or larger tanks. Also, if prefabricated tanks are used, they must remain small enough to permit transport from the fabricating facility to the treatment facility, thus limiting the cross-sectional surface area of the tank.
It is desired that a more efficient scrubber be designed that permits larger volumes of air to be treated over a given period of time in a smaller plot plan. Additionally, easy access into the scrubber must be provided to permit replacement of exhausted carbon. Furthermore, it is desirable to be able to reverse the air path through the scrubber due to process flow changes without modifying any of the internals of the tank. The capacity of the scrubber can be increased by simply increasing the height with the same diameter. The present invention achieves these goals by using an annular carbon bed.
In the case of a cylindrical tank having an annular carbon bed disposed upright inside the tank (see FIGS. 2-4), greater carbon surface area is available than with the same sized prior art tank (see FIG. 1) whenever .pi.d.sub.e h, the cylindrical surface area of the annular carbon bed is greater than .pi.d.sub.t.sup.2 /4, the circular surface area of the prior art carbon bed (where d.sub.e is the effective diameter of the annular carbon bed as described hereafter, h is the height of the bed and d.sub.t is the inside diameter of the prior art tank). Thus, greater volumes of exhaust air can be treated in the same diameter tank. Furthermore, designers may increase carbon surface area by increasing the height and/or diameter of the tank. This choice allows the designer to design the tank to fit a particular restricted location or to use a prefabricated tank having an overall size that is better adapted for transporting to a desired site. Additionally, the layout described herein allows for easy removal of exhausted carbon media and an expedient and uniform method of refilling the scrubber with carbon.